1. Field of the Invention
The present invention relates to a nonaqueous secondary cell.
2. Description of the Prior Art
Recently, owing to the advance in electronic technology, there have been promoted the performance enhancement and miniaturization of electric appliances such as cellular telephones, notebook-type personal computers, video cameras, and the like, and accordingly, there is a strong demand for secondary cells that have high energy densities and can be used in these electric appliances. A representative cell that can meet such a demand is a nonaqueous secondary cell in which lithium salt is used as a negative active material.
A nonaqueous cell comprises, for example, a negative electrode comprising a current collector supporting a carbon material which absorbs and releases lithium ions, a positive electrode comprising a current collector supporting a composite lithium oxide such as a lithium-cobalt composite oxide which absorbs and releases lithium ions, and a separator holding an electrolyte solution dissolving such lithium salts as LiClO4, LiPF6, and the like in an aprotic organic solvent and being interposed between the negative and positive electrodes to prevent short-circuiting of both electrodes.
The positive and negative electrodes are formed in thin sheets or foil shapes, and are piled or wound spirally through a intermediary of the separator to form an electric power generating element. The electric power generating element is housed in either a metallic can made of a stainless steel, a nickel plated iron, or lighter aluminum or a cell container made of laminate film, and subsequently an electrolyte is poured into the cell container, which is sealed for fabricating a cell.
Among a variety of characteristics to be considered such as the charge and discharge characteristics, cycle life characteristics, high-temperature standing characteristics, and the like, the cycle life characteristics capable of suppressing initial-stage performance deterioration over a long period is one of the important characteristics. For example, in a cell in which a carbon material is used as the negative electrode material, the cell characteristics are changed significantly, depending on the kind of the solvent in a nonaqueous electrolyte. It is well known that the electrochemical characteristics of the carbon material can be fully exploited, when such carbonic acid esters as, for example, ethylene carbonate, dimethyl carbonate, vinylene carbonate, or the like is used as a solvent. On the other hand, however, there is a problem that when these solvents are used, the solvents are decomposed while generating gases, and accordingly the cell capacity is gradually lowered with development of the charge and discharge cycles.
In order to solve these problems, a method is proposed in which vinylene carbonate or vinyl ethylene carbonate is added to the electrolyte. For example, in Japanese Patent Laid-Open No. 6-84542, Japanese Patent Laid-Open No. 8-45545, and the like, proposals have been disclosed in which vinylene carbonate is added to the carbonate electrolyte containing ethylene carbonate (EC) as the main component. In Japanese Patent Laid-Open No. 4-87156, a proposal has been made in which vinyl ethylene carbonate is used in a nonaqueous electrolyte cell using metallic lithium for the negative electrode.
Even with these methods, satisfactory cycle life characteristics have not yet been obtained.
On the other hand, the high temperature standing characteristics are an important group of characteristics in a nonaqueous secondary cell, which are assessed by measuring the swelling degree and the discharge capacity of the cell after the cell in a charged state has been allowed to stand for a specified duration in an environment where the temperature is 80° C. or above.
There are available many methods for improving the high temperature standing characteristics, among which are a method in which a solvent having a high boiling point and a low vapor pressure is used, and a method in which the decomposition of the nonaqueous electrolyte on the surfaces of the positive and negative electrodes is suppressed. As in the former case, however, when a solvent having a high boiling point and a high vapor pressure is used, there occur a problem that generally the viscosity of such a solvent is high and the electric conductivity of the nonaqueous electrolyte is lowered, and hence the discharge characteristics of the cell are lowered, and the like. In order to overcome this problem, a method has been proposed in which γ-butyrolactone that has a high dielectric constant and a high boiling point and the like is used (Japanese Patent Laid-Open No. 2000-235868).
γ-Butyrolactone, however, tends to undergo reductive oxidation on the negative electrode while charging, and thereby occurs a problem that the resulting decomposition products cause clogging of the separator and the surface resistance of the negative electrode is increased, leading to a remarkable capacity lowering after repetition of charge and discharge cycles.
For the purpose of suppressing the reductive decomposition of the solvent on the negative electrode, a number of methods have been proposed in which, in order to suppress the reductive decomposition of a lithium salt, a compound which forms a so-called SEI (solid electrolyte interface) on the negative electrode is added to the electrolyte (Japanese Patent Laid-Open No. 2001-6729).
When these SEI forming agents are used, a low-conductivity and high-resistance SEI is formed on the negative electrode, and accordingly there occur problems that the charge and discharge performance of the cell is remarkably lowered, that the swelling of the cell becomes remarkable owing to the elevated internal pressure due to the gas generated by the decomposition of the surplus portion of the added SEI forming agent when the SEI agent is added in excess, and the like.
The present invention has been developed for the purpose of overcoming the above described problems, and an object of the present invention is to provide a nonaqueous secondary cell having excellent charge and discharge characteristics, cycle life characteristics, and high-temperature standing characteristics.